1. Field of the Invention
The present invention relates to an organic electroluminescence device, a light emission apparatus, and a method of fabricating the organic electroluminescence device.
2. Description of the Background
Recently, active research and development have been made on organic electroluminescence devices (hereinafter simply referred to as EL (Electro-Luminescence) devices) that are self-emission type light emitting devices. Active research and development have also been made on light emission apparatuses that use the organic EL devices, such as surface emitting light sources, illumination devices, and display devices.
A typical organic EL device includes an anode, a cathode, and an organic layer that includes an organic light emitting layer and is sandwiched between the anode and the cathode. In the organic EL device, excitons are generated when the recombination of holes and electrons injected from the anode and the cathode occurs in an organic light emitting layer. Light having a wavelength corresponding to an energy band gap is emitted when the excitons return to the ground state.
In order to efficiently inject the electrons and the holes to the organic light emitting layer, a material having a relatively large work function is selected for the anode, and a material having a relatively small work function is selected for the cathode. A transparent conductive metal oxide, such as ITO (Indium Tin Oxide) and IZO (Indium Zinc Oxide, registered trademark), may be used for the anode. On the other hand, a metal, such as aluminum, magnesium, barium, cesium, and silver, or an alloy of such metals, may be used for the cathode.
The transparent conductive metal oxide is formed by vacuum deposition, such as sputtering. Hence, in order to prevent damage to the organic layer, including the organic light emitting layer, by the plasma or radiant heat when the transparent conductive metal oxide is formed, the transparent conductive metal oxide is usually formed before the organic layer. For this reason, the anode, the organic layer including the organic light emitting layer, and the cathode of the organic EL device are successively formed on a substrate in this sequence, as proposed in Japanese Laid-Open Patent Publications No. 9-148066, No. 10-162959, No. 2007-096270, and No. 2009-193774, for example. In other words, a lower electrode forms the anode, and an upper electrode forms the cathode.
The structure in which the lower electrode forms the anode and the upper electrode forms the cathode is suited for a bottom emission type organic EL device that emits the light from the substrate end of the device.
However, when the bottom emission type organic EL device is used for an active drive organic EL display device, a numerical aperture of each pixel becomes limited by a TFT (Thin Film Transistor) circuit or the like formed on the substrate. On the other hand, when the light is to be emitted from the organic EL device end on the opposite end from the substrate, the numerical aperture of each pixel will not be limited because no TFT circuit or the like is provided above the light emitting part. This latter type which emits light from the organic EL device end is referred to a top emission type.
In the top emission type organic EL device, the lower electrode is preferably made of a metal that is optically reflective, and the upper electrode is preferably made of a transparent conductive metal oxide that is transparent. This means that the lower electrode is preferably made of a material having a relatively small work function, and the upper electrode is preferably made of a material having a relatively large work function. In other words, the top emission type organic EL device that emits the light from the organic EL device end preferably has the so-called inverted stacked structure in which the lower electrode forms the cathode and the upper electrode forms the anode, as proposed in a Japanese Laid-Open Patent Publication No. 2010-040512, for example.
However, the following problems may be encountered in the organic EL device having the inverted stacked structure.
For example, when forming the transparent conductive metal oxide by the vacuum deposition such as sputtering in order to form the upper electrode that forms the anode, the organic layer underneath may be damaged during the sputtering to form the transparent conductive metal oxide.
According to the examples proposed in the Japanese Laid-Open Patent Publications No. 9-148066, No. 10-162959, and No. 2009-193774, the upper electrode is encapsulated by providing a buffer layer or a protection layer on the upper electrode by the vacuum deposition, in order to protect the upper electrode forming the cathode of the organic EL device having a structure other than the inverted stacked structure. However, if the buffer layer or the protection layer is provided between the organic layer and the upper electrode forming the anode in the organic EL device having the inverted stacked structure, it becomes difficult to inject the holes from the anode to the organic light emitting layer. Consequently, the luminous efficacy deteriorates, and a driving voltage required to drive the organic EL device increases.
On the other hand, according to the example proposed in the Japanese Laid-Open Patent Publication No. 2010-040512, the upper electrode forming the anode of the organic EL device is formed by printing a layer of conductive polymer, such as polyaniline and polythiophene, in place of the transparent conductive metal oxide. However, because conductive polymers generally absorb light in a visible region, it is difficult to increase the transmittance of the anode in this example. Furthermore, the conductive polymers have a small conductivity compared to that of the transparent conductive metal oxide, such as the ITO, and thus, it is difficult to increase the conductivity of the anode in this example.